A flow cytometer is one type of device used for analyzing the kind, size, shape, structure and other characteristics of microparticles, such as cells, microorganisms or microbeads. In the flow cytometer, microparticles arranged in a row are passed through a flow channel. When passing through a specific section of the channel, the microparticles are irradiated with light. Then, the light that originates from those microparticles (e.g. scattered light or fluorescent light) is detected to determine a characteristic of each microparticle (see Non Patent Literature 1).
For example, in the case of detecting a fluorescence emitted from a cell, the cell labelled with a fluorescent dye is irradiated with excitation light (e.g. laser light), and the fluorescence emitted from the fluorescent dye due to the irradiation is detected through a wavelength-selecting device, such as a bandpass filter or dichroic mirror. In this method, a plurality of fluorescent emissions in different wavelength bands can be detected by using a plurality of wavelength-selecting devices with different wavelength selection bands and providing a separate photodetector for each wavelength-selecting device to detect the thereby selected light. Accordingly, if a fluorescent dye having a different fluorescence wavelength band is used for the labelling of each kind of cell, it is possible to determine optical characteristics of each kind of cell.